qmake5_base: define both QMAKESPEC and let recipe select which one to use
qmake5_base: define both QMAKESPEC and let recipe select which one to use

* different platform/xplatform can enable cross-compile build in upstream qmake
  but that's not the same as what recipes are doing (upstream builds native tools
  when bootstraping target qtbase, recipes build native tools with separate
  qtbase-native and then want to skip building tools)
* still separate variables for both QMAKESPECs can be useful e.g. for
  other native recipes

Tofee/meta-qt5,Ivan-SB/meta-qt5,schnitzeltony/meta-qt5,sapiippo/meta-qt5,sapiippo/meta-qt5,timopulkkinen/meta-qt5,timopulkkinen/meta-qt5,opendreambox/meta-qt5,timopulkkinen/meta-qt5,schnitzeltony/meta-qt5,kraj/meta-qt5,cjp256/meta-qt5,cjp256/meta-qt5,jruffin/meta-qt5,Ivan-SB/meta-qt5,timopulkkinen/meta-qt5,opendreambox/meta-qt5,Tofee/meta-qt5,meta-qt5/meta-qt5,kraj/meta-qt5,meta-qt5/meta-qt5,opendreambox/meta-qt5,jruffin/meta-qt5